The present invention relates to an apparatus for coupling energy in a cavity resonator to a microstripline circuit, and more particularly, to an apparatus for variably coupling such energy.
Cavity resonators and microstripline circuits are well know in the art of employing high frequency electromagnetic energy. A cavity is a hollow conductive circuit sometimes having a rectangular box-like shape and is typically used as a frequency resonant element. A microstripline circuit is used to propagate electromagnetic energy and consists of a ground plane and a foil strip separated by dielectric material. Although microstripline circuits are more subject to radiation losses than are other transmission structures, such as waveguides, they may be inexpensively constructed by familiar photo etching techniques. Moreover, microstripline circuits may be interfaced quite easily with a variety of electronic components using minimal circuit board real estate.
In many systems, such as point-to-point radio communication systems, it is necessary to interface energy in a resonant structure to various portions of the system. There are a number of techniques that perform this interface.
One example is a commercially available microwave duplexer in which resonant energy is coupled from a resonant structure to external circuitry using a metal rod affixed with a dielectric sleeve in a metal bushing which is mounted perpendicular to a wall and partially extending into the resonant structure. The electromagnetic energy is coupled to the metal rod and out through a coaxial cable attached thereto. A critical aspect of such a design is the availabilty to adjust the coupling such that the Q of the resonant structure coupled through the probe may be set according to desired specification. To accomplish this task, one of two procedures may be used. The first procedure involves turning the bushing in the structure until the desired Q is obtained. However, changing the depth of the bushing can significantly alter the resonant frequency itself.
The second procedure involves trimming the length of the metal rod. This requires removing the metal bushing from the cavity, trimming the rod, reinserting the bushing, and measuring for the desired Q. If the Q is not found to agree with specification, the procedure must be repeated. Not only is this second procedure overly cumbersome, but a replacement rod is required if the metal rod is trimmed too far.
There are still other techniques known in the art which utilize microstripline circuits to couple energy from a waveguide to external circuitry. One such example is described in Murphy--U.S. Pat. No. 4,453,142, assigned to the same assignee of the present invention. Murphy describes a microstripline waveguide transition which uses the microstripline to extract the energy from the waveguide. The microstripline is mounted at a right angle on a wall of the waveguide. The microstripline is preformed into a transition section and a probe section. Energy in the waveguide is coupled to the probe and onto the external microstripline through the transition section. The transition section width is formed as narrow as possible to minimize capacitive coupling to the waveguide wall and is limited to a length of an integral multiple of one-half the wavelength for a smooth impedance match from the probe to the microstrip. Although this invention alleviates certain problems discussed therein, it requires very detailed probe manufacturing to obtain a given coupling. Furthermore, this kind of transition is not practical for cavity resonators which are tuned over a wide range of resonant frequencies since it cannot be adjusted.
What is needed is a cavity to microstripline transition which can easily be adjusted to couple the required amount of high frequency energy to microstripline circuitry.